A significant portion of the product design cycle of many modern manufactured products is the time required to build and test prototype parts. Fast turn-around machine or prototype shops are used to produce prototype parts useful in the development of the manufactured product. One popular method of producing prototype parts in such shops is the subtractive machining of a block of material until the part matches the dimensions in a mechanical design. As is well known, the accuracy with which the machined prototype part matches the design can widely vary, primarily according to the skill of the machinist. In addition, the ability of subtractive processing to produce parts of complex shape is limited, and the time required for the machining of the part can be quite lengthy. As such, the product design cycle depending upon subtractive machining of prototypes is often lengthy, delaying the time-to-market of the eventual manufactured product.
Accordingly, new methods for the producing of parts, especially prototypes, have been developed in recent years to enable the rapid manufacture of complex parts directly from computer-aided-design (CAD) data bases. A particularly successful and recently developed additive process is commonly referred to as selective laser sintering. According to the selective laser sintering process, a laser is scanned in raster fashion over a layer of fusible powder, and modulated on and off, to fuse selected portions of the layer according to a cross-section of the desired part. After the fusing of the desired portions of a layer, another layer of powder is placed and similarly selectively fused, with fused portions of the later layer fusing to fused portions of the previous layer. Continued layerwise processing in this manner results in a part which can be quite complex in the three-dimensional sense.
The selective laser sintering process is described in detail in application Ser. No. 07/951,349, filed Sep. 25, 1992, now U.S. Pat. No. 5,296,062 and in the above-referenced U.S. Pat. Nos. 5,076,869, issued Dec. 31, 1991, and 4,944,817 issued Jul. 30, 1990, all assigned to Board of Regents, The University of Texas System, and incorporated herein by this reference. The selective laser sintering method is also described in U.S. Pat. No. 4,863,538, issued Sep. 5, 1989, U.S. Pat. No. 5,017,753 issued May 21, 1991, and U.S. Pat. No. 4,938,816 issued Jul. 3, 1990, all also assigned to Board of Regents, The University of Texas System and incorporated herein by this reference, and in U.S. Pat. No. 4,247,508, assigned to DTM Corporation, also incorporated herein by this reference.
By way of further background, U.S. Pat. No. 5,156,697 issued Oct. 20, 1992, U.S. Pat. No. 5,147,587 issued Sep. 15, 1992, and U.S. Pat. No. 5,182,170 issued Jan. 26, 1993, all also assigned to Board of Regents, The University of Texas System and incorporated herein by this reference, as well as the other referenced U.S. Patents noted hereinabove, describe the selective laser sintering of various materials and combinations of materials such as plastics, waxes, metals, ceramics, and the like. In particular, the selective laser sintering method has been especially beneficial in the production of molds or cores useful in investment casting. For example, a part formed of a low-temperature wax by the selective laser sintering process may be used in the well-known "lost wax" method of forming an investment casting mold.
The above-referenced U.S. Pat. Nos. 5,156,697 and 5,182,170 each describe selective laser sintering methods for producing parts from high temperature materials, and thus which may be useful in directly forming an investment casting mold. In the processes described in each of these patents, the laser irradiation either causes or facilitates a localized chemical reaction at the irradiated location of the powder, either among powder constituents or between the powder and the surrounding atmosphere. The processes described in these patents are referred to as selective laser reactive sintering ("SLRS").
For purposes of the description herein, the term "selective laser sintering" is intended to include selective laser reactive sintering, as well as selective laser sintering where the fusing mechanism is sintering of the liquid-phase (i.e. in which localized melting of the powder occurs) or solid-phase type.
By way of further background, another method for producing parts of high temperature materials, such as high temperature ceramics and ceramic composites, utilizes a powder of polymer-coated ceramic, such as described in the above-referenced U.S. Pat. Nos. 5,076,869 and 4,944,817. As described in these Patents, after the formation of a part by flowing the polymer coating to bind particles of a high temperature material, the part is subjected to a post-process anneal in which the polymer coating dissociates and the remaining particles of the high-temperature material sinter together and form the part.
By way of still further background, application Ser. No. 07/854,246, filed Mar. 20, 1992, now U.S. Pat. No. 5,284,695 assigned to The University of Texas System, and incorporated herein by reference, describes another selective laser sintering method which forms a "green" part, or preform, from a spray-dried powder of ceramic or metal particles coated with a polymer. According to this method, because the high temperature particles are incompletely coated with the polymer in the spray drying process, subsequent impregnation of the green part with a binder fuses the high-temperature particles into an object of the same shape as the green part. Annealing of the part is then performed to drive off the polymer and yield a high temperature part defined by low temperature selective laser sintering.
By way of further background, several synthesis methods are well known in the art for preparing nanocomposite materials. These known methods include sol-gel processing, evaporation-condensation-decomposition methods, and high-energy ball milling, among others. The term "nanocomposite" conventionally refers to multiphase solids where the constituent particles are of a size of on the order of one to one hundred nanometers in diameter; in contrast, the terms "nanophase" and "nanocrystalline" refer to materials that are single phase solids of nanometer size.
It is an object of the present invention to provide a method of forming objects by selective laser sintering, where the objects may be formed of a high temperature material such as a ceramic or a metal.
It is a further object of the present invention to provide such a method using a powder in which sintering is enhanced in the selective laser sintering process or in a post-process cure.
It is a further object of the present invention to provide such a method where the resulting object is formed with a high density, and is therefore less vulnerable to defects such as microcracks, undesirable porosity, and distortion upon densification.
Other objects and advantages of the present invention will become apparent to those of ordinary skill in the art having reference to the following description together with the drawing.